Throughout the text, “mineral particle” denotes any inorganic particle which contains carbon, where applicable, only in the form of carbonate or cyanide.
Throughout the text, “hydrothermal treatment” denotes any treatment carried out in a closed vessel, such as an autoclave, in the presence of water, at a predetermined temperature and at a pressure greater than atmospheric pressure.
Many minerals such as borates or silicates are used in various industrial fields. Phyllosilicate mineral particles, such as talc, are used, for example, in the form of fine particles in many industrial sectors, such as: thermoplastics, elastomers, paper, paints, varnishes, textiles, metallurgy, pharmaceuticals, cosmetics, phytosanitary products or fertilizers in which phyllosilicates such as talc are used, by being incorporated into a composition, as an inert filler (for their chemical stability or for the dilution of expensive active compounds) or as functional fillers (for example to enhance the mechanical properties of some materials).
Natural talc, which is a hydrated magnesium silicate of the formula Si4Mg3O10(OH)2, belongs to the phyllosilicate family. The phyllosilicates are composed of an irregular stack of elementary lamellae of crystalline structure, the number of which varies from several units to several tens of units. Among the phyllosilicates (lamellar silicates), the group comprising especially talc, mica and montmorillonite is characterized by the fact that each elementary lamella is constituted by the association of two tetrahedral layers situated on either side of an octahedral layer. This group corresponds to the 2:1 phyllosilicates, which include especially the smectites. In view of their structure, the 2:1 phyllosilicates are also described as being of the T-O-T (tetrahedron-octahedron-tetrahedron) type.
The octahedral layer of the 2:1 phyllosilicates is formed of two planes of O2− and OH− ions (in the molar proportion O2−:OH− of 2:1). On either side of this middle layer there are two-dimensional lattices of tetrahedrons, of which one of the vertices is occupied by an oxygen of the octahedral layer, while the other three are occupied by substantially coplanar oxygens.
As regards talc, a high purity, fineness of the particles and good crystal properties are desirable for many of its applications, because they are determinative for the quality of the end product.
However, the preparation of a pulverulent composition from blocks of natural talc by grinding and treating the talc does not allow the purity, the lamellarity and the size of the particles of talc obtained to be controlled. Furthermore, grinding of natural talc irreversibly causes amorphization thereof, or at least a considerable reduction in its crystallinity.
Within this context, WO 2008/009799 proposes a process for preparing a talcose composition comprising at least one synthetic mineral containing silicon, germanium and metal of the formula (SixGe1-x)4M3O10(OH)2 by hydrothermal treatment of a hydrogel containing silicon and/or germanium, and metal of the formula (SixGe1-x)4M3O11, n′H2O, in the liquid state, at a temperature of from 300° C. to 600° C. Such a talcose composition according to WO 2008/009799 comprises particles containing silicon, germanium and metal of the formula (SixGe1-x)4M3O10(OH)2 which have structural similarities with natural talc and have, in X-ray diffraction, some diffraction lines characteristic of talc, that is to say lines corresponding to the planes (001), (020), (003) and (060).
Accordingly, although a process according to WO 2008/009799 permits the preparation of a synthetic mineral that is similar to talc and the purity of which can be controlled, it is hardly compatible with high industrial demands in terms of efficiency, profitability and structural qualities of the synthetic mineral particles obtained.